The present invention provides an improved pressure swing adsorption (PSA) process capable of delivering high purity and high recovery of a high purity gas, such as argon, from a feed stream. More specifically, the present invention provides an improved process with high recovery for purification of crude argon available from a cryogenic air separation unit. The high recovery enables the present invention to become a complete process without any additional requirement for purification and recycle back to the cryogenic air separation unit.
Currently, oxygen and nitrogen, the two main products of an air separation, can be directly removed from a two-stage cryogenic rectification unit involving a high pressure column and a low pressure column. Argon, which constitutes almost 1% of the feed air, is then enriched in the middle section of the low pressure column. This enriched argon containing about 10 to 12% of argon, 0.1% of nitrogen and the rest of oxygen is fed to the argon low ratio column to produce crude argon containing impurities of about 1 to 5% of oxygen and 1 to 3% of nitrogen. Crude argon is then purified to about 99.999% purity, typically first by catalytic deoxygenation or by a superstaged argon column to remove oxygen, then by rectification in a high ratio column to remove nitrogen.
Catalytic deoxygenation requires the availability of hydrogen, which is not always available and cost effective everywhere in the world. Hydrogen reacts with oxygen to form water, which is then removed from crude argon. Superstaging is another alternative for oxygen removal by adding additional separation stages in the argon column. However, the number of these additional stages could be fairly large, for example, between 115 to about 140, because of the small difference in the relative volatility between oxygen and argon. Furthermore, a high ratio cryogenic column will still be required for additional nitrogen removal if nitrogen is present in the crude argon column.
As compared to the above conventional very elaborate methods of recovering 80 to about 90% argon from air, a PSA process provides a simple and effective alternative for argon purification and recovery. No hydrogen or additional cryogenic stages are required. However, conventional PSA processes suffer from a rather low argon recovery of about 40%. Thus, it is necessary to recycle the PSA waste stream, still containing significant amount of argon, back to the cryogenic air separation unit for additional recovery. Consequently, PSA is much less attractive.
High purity argon is generally produced by purifying crude argon available from an air separation unit. Adsorption is a promising alternative to cryogenic superstaging as disclosed by Bonaquist and Lockett in U.S. Pat. No. 5,440,884 and catalytic deoxygenation as disclosed by Tomita et al. in U.S. Pat. No. 5,783,162.
Jain and Stern in U.S. Pat. No. 5,601,634 and Jain and Andrecovich in AU-A-47537/93 disclose respective cryogenic temperature swing adsorption purification processes. In AU-A-47537/93, the cryogenic TSA is carried out below 150 K in a two layer adsorbent bed. The first layer comprises one or more equilibrium selective adsorbents, such as calcium exchanged type X and A zeolite to preferentially adsorb nitrogen over argon. The second layer comprises one or more rate selective adsorbents, such as CMS and 4A type zeolite, to preferentially adsorb oxygen. Upon completion of adsorption, the bed is regenerated by passing a warm purge gas substantially free of impurities, such as nitrogen and oxygen. This prior art involves low temperature adsorption and argon recycle.
U.S. Pat. No. 5,601,634 discloses a cryogenic TSA process with a liquid-vapor two phase feed. The adsorption bed contains one or more adsorbents selective for nitrogen and/or oxygen at a temperature between the bubble and dew point of the two phase mixture. The advantage of this two phase feed is that any increase in temperature during the adsorption step will evaporate some of the liquid and that the heat of adsorption is offset. This can improve the adsorption capacity. However, because of low operating temperature and the warm purge required, this process is energy relatively intensive.
Nguyen et al. in U.S. Pat. No. 5,730,003, teaches a PSA process for crude argon purification. The process uses oxygen rate or equilibrium selective adsorbent for oxygen removal. In Nguyen et al., the O2 rate selective adsorbents include CMS, clinoptilolite, type A zeolite, and the O2 equilibrium selective adsorbents include adsorbents disclosed by Ramprasad et al. in U.S. Pat. No. 5,294,418. A layer of nitrogen equilibrium selective adsorbent such as CaA and type X zeolite is mentioned for nitrogen removal. The process uses the following cycle steps: feed pressurization, adsorption, cocurrent depressurization, countercurrent blowdown, countercurrent purging and product pressurization. This process does not require low temperature as required by a TSA. However, the argon recovery is low (about 40%) and recycling of desorption gas, during bed regeneration, back to the cryogenic air separation plant is necessary to enhanced argon recovery. This prior art uses a Simplex two bed PSA system.
Kumar et al. in U.S. Pat. No. 4,477,265, discloses a two stage PSA process for argon purification. The two layers of adsorbents for oxygen and nitrogen removal are in two separated stages. The two stages are connected in series. This allows the process to be more flexible, for example, it permits possible bed interactions even within a stage and using different number of beds in different stages. In one preferred embodiment, three beds are in fact used in the first stage for nitrogen removal using a nitrogen equilibrium selective adsorbent. Two beds are in the second stage for oxygen removal using an oxygen rate selective adsorbent. The basic cycle steps include adsorption, evacuation and pressurization. Also, argon recovery is low, and recycling the waste stream, still containing considerable amount of argon, back to cryogenic unit is necessary for additional recovery. In addition, recycling of PSA waste stream back to the cryogenic plant makes the air separation unit more complex and a PSA option less attractive.
Also, Kumar et al. in U.S. Pat. No. 5,395,427 discloses a two stage PSA process using oxygen and nitrogen equilibrium selective adsorbents for producing high purity oxygen from air. The oxygen equilibrium selective adsorbent is a cobalt-based material and preferably used in the second stage. Carbon dioxide, water and nitrogen are preferably removed in the first stage filled with one or more adsorbents selective for the impurities. Oxygen is desorbed from the second stage as product, and the effluent is used to regenerate the first stage adsorbent(s).
Wilson, U.S. Pat. No. 4,359,328, discloses an inverted PSA process, in which a strong component is adsorbed at low pressure while a weak component is adsorbed at high pressure. This is just opposite to conventional PSA process and could be used to recover strong component with enhanced purity.
Lee and Paul, U.S. Pat. No. 5,738,709, discloses a nitrogen PSA process with an intermediate pressure transfer. Instead of a conventional end-to-end (bottom, top or both) transfer, a transfer is carried out from an intermediate point of the high pressure bed to a point close to the feed end of the low pressure bed. Such a transfer increases the productivity and recovery of nitrogen.
Leavitt, U.S. Pat. No. 5,085,674, discloses a Duplex PSA process. The setup is similar to a two stage PSA but with two distinguished features: intermediate feed between the two stages rather than at one end (top or bottom) and recycling capability from the low pressure bed to the high pressure bed. Such a process combines both the conventional PSA and the inverted PSA features of U.S. Pat. No. 4,359,328, and could provide high purity and also recovery. However, this process has not been applied to argon purification with removal of both oxygen and nitrogen. In addition, the process does not advantageously use the capability of the system, e.g., intermediate pressure transfer.
Diagne et al, J. Chem. Eng. Japan, 27, 85 (1994), Ind. Eng. Chem. Res., 34, 3089 (1995), J. Chem. Tech. Biotechnol. 65, 29 (1996), discloses a Duplex process for carbon dioxide removal and enrichment from air-carbon dioxide mixtures. It simultaneously concentrates and removes carbon dioxide beyond the factor of pressure ratio between adsorption and desorption, a limit for the conventional PSA.
Garrett, U.K. Patent No. 2,174,922 A, discloses a fast cycle PSA system for separating a gas feed mixture into two gas streams. The system is close to the Duplex and also has intermediate feed and bottom recycle features. However, the process focuses on fractional pressure change for thermodynamic efficiency and short cycle time. This consequently limits the purity in the separation process. Therefore, it could not be applied to purification where high purity is required as in the instant invention.
The non-PSA processes, i.e., superstaged cryogenic distillation and catalytic deoxygenation, in the prior art of crude argon purification require a large number of cryogenic stages or the availability of hydrogen. In addition, they normally require some recycling between argon purification unit (argon column or deoxygenation system) to the main air separation unit and further residual nitrogen removal. Therefore, the entire hybrid plant from air separation to argon purification becomes very complex, less flexible and less attractive.
On the other hand, conventional PSA processes in the prior art could provide high argon purity, but the recovery is limited. So, a recycling from PSA back to the air separation unit is normally required for additional argon recovery. This in turn makes PSA integration more difficult and less flexible.
The Duplex process of Leavitt in U.S. Patent No. 5,085,674 is able to provide both high purity and recovery by using an intermediate feed and a recycle at the bottom ends between desorption and adsorption beds. In the Leavitt process, argon is purified from its mixture with about 100 ppm nitrogen using 13xc3x97 molecular sieve and operating at 105-210 kPa pressure range and at ambient temperature. However, Leavitt""s Duplex process does not utilize simultaneous removal of oxygen and nitrogen, and thus does not provide a complete argon purification process.
It is an object of the present invention to provide an advanced PSA purification process which is capable of delivering a gas, such as argon, at high purity and high recovery. More specifically, it is an object of the instant invention to provide an improved crude argon purification process which does not require any additional purification or recycling from PSA back to the cryogenic air separation plant for higher argon recovery. In addition, the instant invention is intended to use more efficient process cycle and adsorbents.
In a preferred embodiment, the instant invention preferably provides an improved Duplex process with simultaneous removal of both oxygen and nitrogen. Therefore, it is a complete argon purification process, and there is no need for recycle from PSA to the cryogenic unit. In addition, in a preferred embodiment, the instant invention enhances process performance and economics by using improved adsorbents (e.g., LiX zeolite with SiO2/Al2O3 ratio of 2.0-2.5, CMS and materials disclosed by Ramprasad et al. in U.S. Pat. No. 5,294,418) and process cycle (short cycle time and overlapping steps). Additionally, in a preferred embodiment, the instant invention uses improved bed-to-bed interactions such as dual end pressure transfer.
In a more preferred embodiment, starting with crude argon (e.g., 97.5% Ar, 1.5% O2 and 1% N2) from a cryogenic air separation plant, the instant invention can purify argon to over 99.999% purity and with high recovery over 70% (theoretically as high as 99%) while employing only a Duplex PSA system with no recycle requirement of the argon in the PSA waste stream to the cryogenic air separation unit.
An improved process of this invention is characterized by one or more of the following:
a Duplex PSA system for crude argon purification with simultaneous removal of oxygen and nitrogen;
high purity and recovery of argon product;
a complete purification system: no need for recycle from the PSA to the cryogenic air separation unit and further purification;
use of advanced materials such as nitrogen selective (e.g., LiX) and oxygen equilibrium selective adsorbent (e.g., TEC); and
improved process cycle: overlapping steps, fast cycle and bed-to-bed interaction.